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Geology

The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages, and a few new episodes were posted. Now, the blog/podcast is on a weekly schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Thursday, December 25, 2014

December 25. Quaternary Megafauna Extinctions

Mammoths, mastodons, saber-toothed cats, American horses and camels, the giant bear, the Irish elk, Glyptodonts, which were giant relatives of the armadillo, the Megatherium, an elephant-sized ground sloth, giant kangaroos, a 7-foot-long beaver, and dozens of other large animals and animal groups, collectively called megafauna, died off during the Quaternary Period of the Cenozoic Era, many of them at about the time of glacial retreat, 10,000 to 12,000 years ago at the end of the Pleistocene Epoch. Some of them were relatively new to planet earth, and some species had been around for millions of years.

Don’t visualize this extinction as an instant in time, or really even a short period of a few hundred years. It’s true that there seems to be a spike in extinctions right at the time the glacial period was ending, 10 or 12 thousand years ago, but many animals, including for example the giant terror birds of South America, were becoming extinct at about the beginning of the Pleistocene glaciation, 2.5 million years ago.

And how do we know something was completely extinct? We can only go by the fossil record, and fossils of modern large animals are rare. Even for a particular species, extinction can take thousands of years, winding down to the last mastodon, or passenger pigeon, or whatever. Mammoths got started about five million years ago, in the Pliocene, and were mostly gone at about the end of the glacial period, 12,000 years ago. But small populations survived on isolated Arctic islands such as St. Paul, off Alaska, and Wrangel, off Siberia, as recently as 3700 years ago. Different species in different niches disappeared sooner or later, depending on circumstances. The pygmy mammoth probably survived on the Channel Islands of California until about 11,000 years ago.

So, why did the mammoths, giant sloths, and all the rest die? The first obvious explanation is the dramatically changing climate. Glacial environments certainly constrain food supplies and habitat, and other habitats, even in temperate zones, change. That could account for some of the extinctions early in the Pleistocene, 2 to 2½ million years ago. But why would so many go extinct at the end of the glaciation? Well, change is change, and warmer might not necessarily always be better for all species; forests supplanted grasslands in the north, perhaps denying mammoths their main food sources. It might be a simple matter of inability to adapt to the changes. Once a population becomes small, many things – disease, a big storm, a flood – could have been the final blow that ended a species that may have been in decline for some time.

The second primary idea for the extinction of the Quaternary megafauna is still somewhat controversial, and involves hunting of the animals by predators – specifically, early humans. There’s no question that humans hunted and ate some of these animals – or hunted them to remove them as threats. Homo erectus was killing mammoths a million and a half years ago. The question really is, were humans the primary, or even a major, cause of the extinctions.

There’s pretty good correlation between the arrival and expansion of humans in various areas with the demise of the large mammals. The correlation in time and space alone does not mean the one caused the other. Arguments against this hunting hypothesis include the idea that the small populations of humans would not likely have been enough to eradicate millions of animals. And while the correlations are best in the Americas and Australia, the extinctions take place at about the same time in Eurasia, where these animals had been exposed to human threats for far longer.

The bottom line is that the ultimate cause of extinction of so many large mammals is uncertain. Humans may have had a role, either large or small, and climate factors are likely to have been important, and perhaps even controlling in the extinctions. When you take multiple climatic factors into account, including temperature changes, precipitation patterns, vegetation and habitat changes, maybe that’s enough to explain most of the extinctions, and humans or disease or something else were simply the exclamation point at the end. In such complex systems as the earth, it’s almost impossible to point to any single thing as THE cause, and as usual, the most likely answer is a combination of many factors. We really don’t know.

The intro music is from "Vintage Education" by Kevin MacLeod; public domain from freepd.com. Banner photos by Richard Gibson unless credit line is given. Then, they are either public domain or are used with permission of the photographer.